In an internal combustion engine, a cylinder head gasket is generally provided between the cylinder block and the cylinder head and can have openings aligned with the power cylinders of the engine st serve as a seal for the coolant circulation path and for the lubricant circulation path since both the cooling liquid and liquid oil are circulated through the engine block and cylinder head.
Secondly, it must serve as a seal against the escape of combustion gases which are generated at elevated pressures and temperatures within the engine.
Thirdly, it must serve as a seal against the escape of gas-fuel mixtures or flame fronts so as to maintain the pressure of combustion or work expansion within the cylinders.
Fourthly, the cylinder head gasket generally serves as a controlled-thickness spacer between the cylinder head and the engine block, as a support for the cylinder head and the engine block and in part as an alignment member for properly positioning a cylinder head on the engine block.
Finally, in addition to the mechanical support and positioning functions described, the gasket must be capable of compensating for geometric irregularities of the contact surfaces of the engine block and the cylinder head with the seal so that it maintains the initial sealing action in spite of manufacturing irregularities and in spite of deformations of the engine which may occur in the course of operation and with time.
In the past, the different types of sealing actions have resulted in resort to different solutions for the different seals. For example, sealing of the cooling liquid, e.g. a water-glycol mixture, and the lubricating oil was generally accomplished using an asbestos seal in which asbestos fiber layers were applied to a perforated sheet metal support or spine, by a metal fabric received between two fibrous layers of asbestos and elastomer fiber mixtures, or the like.
The pressure sealing in the engine of the cylinders, i.e. the refractory seal, was generally accomplished utilizing refractory sealing rings of metal such as steel, galvanized steel, steel clad with aluminum or the like.
The conventional cylinder head gasket was thus a composite structure which was clamped tightly in place by the tightening of cylinder head bolts.
In most cases the cylinder head gasket had to be clamped so strongly by such bolts, for effective sealing, that deformation of the cylinder head or other parts of the engine resulted simply from the bolt-tightening operation.
Another disadvantage of the earlier systems was that the handling of the asbestos materials created an environmental hazard and particular danger to workers with the product.
To solve in part the problem with fluid sealing in such gaskets, it is known, moreover, to provide elastomeric seals, particularly of silicon elastomers. In general, however, these have not proved to be fully satisfactory.
Another problem has been created by the tendency to utilize, for Diesel engines having precombustion chambers, cylinder blocks designed primarily for gasoline engines. In these cases, the number of cylinder head bolts which can be used is reduced because of the presence of precombustion chambers; sealing problems result.
Note should also be taken of the fact that with conventional composite cylinder head gaskets utilizing asbestos fibers, for example, a part of the tightening force, generally of the order of 50%, is absorbed by the material constituting the gasket. In the case of metallic gaskets a portion of the force is absorbed by the core structure of the gasket while another portion is expended in crushing the refractory seal. As a result, truly effective sealing requires especially strong tightening of the bolts which leads to temporary or permanent deformations of the various parts involved, namely, the cylinder block, the cylinders, the cylinder head, casings or jackets of the system, and especially cylinder head or other parts which may be composed of lightweight materials such as aluminum.